1. Field of the Invention
The present invention relates to a color cathode-ray tube, particularly to a color cathode-ray tube including a shadow mask that is stretched with a tension applied thereto in one direction.
2. Related Background Art
In a color cathode-ray tube, an electron beam emitted from an electron gun irradiates a phosphor screen formed on an inside surface of a face panel, so that a desired image is displayed. A shadow mask that functions as a color selecting electrode is provided on an electron gun side of the phosphor screen, with a predetermined distance therebetween. The shadow mask is made of a metal plate, in which a multiplicity of electron-beam-passing apertures, each having a rectangular shape (slot shape), are arrayed to allow an electron beam to pass therethrough and impinge on a phosphor at a desired position. Such a shadow mask is held in a state of being stretched.
A deviation of positions of the electron-beam-passing apertures of the shadow mask relative to positions of the phosphors on the phosphor screen causes the electron beams to irradiate a phosphor different from a desired one (this phenomenon is called “mislanding”), thereby causing image quality deterioration, which is called color shift.
One of causes of the mislanding is a phenomenon of thermal expansion of a shadow mask when heated by an electron beam, that is, the so-called doming. To prevent this, the shadow mask is stretched with a tension applied thereto in one direction, so that the thermal expansion is absorbed.
However, the shadow mask stretched with a tension applied thereto tends to vibrate when vibrations or impacts such as vibrations from speakers are transmitted from outside to the shadow mask, and to incur degraded attenuation of the vibrations. Therefore, a display screen tends to sway or become blurred.
One method for attenuating the vibration of the shadow mask is disclosed in JP2000-77007A. The method is described below, with reference to FIG. 7.
FIG. 7 is a schematic perspective view of a mask structure composed of a shadow mask 120 and a frame 130 for framing the shadow mask 120 while stretching the same.
The frame 130 is formed by bonding two pairs of rod-type members into a rectangular frame shape. The shadow mask 120 is made of a flat plate material in an approximately rectangular shape, in which a multiplicity of electron-beam-passing apertures 122 through which an electron beam is to pass are arrayed regularly in an X axis direction and a Y axis direction. The shadow mask 120 is held in a state of being welded to one side of each of supporting members 131a and 131b that form long sides of the frame 130, with a tension T in a direction of a shorter side of the frame 130 being applied to the shadow mask 120.
A plurality of pairs of perforations 125 are formed in end regions of the shadow mask 120, the end regions being at ends of the shadow mask 120 in a direction perpendicular to a direction in which the tension T is applied, in a manner such that each pair of the perforations 125 is arranged in the direction in which the tension T is applied. A vibration suppressor 140 formed by bending a wire-rod into a rectangular frame form is inserted through each pair of the perforations 125 with play.
Such a mask frame is housed in a color cathode-ray tube, arranged so that the direction of the tension T coincides with a vertical direction.
When the shadow mask 120 vibrates, the vibration suppressor 140 moves independently from the shadow mask 120, while coming into contact with, rubbing against, and separating from the periphery of the perforations 125 in the shadow mask. The vibration energy of the shadow mask 120 is consumed by friction caused by such a movement of the vibration suppressor 140 relative to the perforations 125 of the shadow mask 120. Thus, the vibration suppressor 140 functions as a vibration attenuator for attenuating the vibration of the shadow mask 120.
However, the foregoing conventional vibration attenuating method has a problem in that in the case where the same vibrations (or impacts) are applied to a shadow mask, a vibration decay time is not stabilized, and this increases the mean decay time.
The causes of this problem were analyzed in detail, and the following phenomenon was confirmed.
FIG. 8A is an enlarged front view of a portion where the vibration suppressor 140 as a vibration attenuator is attached, and FIG. 8B is a cross-sectional view of the portion taken along an arrow line 8B—8B shown in FIG. 8A, viewed in a direction indicated by the arrows. As shown in the drawing, the vibration suppressor 140 bent in the approximate rectangular frame form is provided with play through a pair of the perforations 125a and 125b, which are formed apart in the vertical direction of the shadow mask 120. The vibration suppressor 140 is attached in a manner such that both ends of the wire-rod bent into an angular U shape are inserted into the pair of perforations 125a and 125b, and then, the both ends are bent back. Here, in some cases, errors occur at bent positions of the vibration suppressor 140. For instance, as shown in FIGS. 8A and 8B, the vibration suppressor 140 sometimes is attached in a manner such that most of a weight of the vibration suppressor 140 is borne by only a surrounding of the lower perforation 125b. In this case, an upper end of the vibration suppressor 140 is stabilized in an inclined state, the inclination being in either one of directions indicated by arrows 142 within a plane parallel with a surface of the shadow mask 120, as shown in FIG. 8A, and in either one of directions indicated by arrows 143 within a plane perpendicular to the surface of the shadow mask 120, as shown in FIG. 8B. When the shadow mask 120 vibrates in this state, the vibration suppressor 140 also floatingly moves in the directions indicated by the arrows 142 and 143. Therefore, the upper bent portion 140a of the vibration suppressor 140 sometimes comes into contact with or rubs against the surrounding of the upper perforation 125a, and sometimes does not. When the upper bent portion 140a of the vibration suppressor 140 is in contact with or rubs against the surrounding of the upper perforation 125a, the effect of attenuating the vibration of the shadow mask 120 is increased. Otherwise, it is decreased. Consequently, the vibration decay time varies, and as a whole, the mean decay time increases.
In contrast to the foregoing, the vibration suppressor 140 is attached in a state of being hung from the upper perforation 125a in some cases, as shown in FIGS. 9A and 9B. When the shadow mask 120 vibrates in this state, a lower end of the vibration suppressor 140 floatingly moves in a direction indicated by an arrow 144 within a plane parallel with a surface of the shadow mask 120 as shown in FIG. 9A, and in a direction indicated by an arrow 145 within a plane perpendicular to the surface of the shadow mask 120 as shown in FIG. 9B. In this case as well, therefore, the lower bent portion 140b of the vibration suppressor 140 sometimes comes into contact with or rubs against the surrounding of the lower perforation 125b, and sometimes does not. Consequently, the vibration decay time varies, and as a whole, the mean decay time increases.
As described above, with the conventional vibration attenuating method employing the vibration suppressor 140, it is difficult to achieve a desired vibration attenuating effect stably due to a relative dimension error between bent positions of the vibration suppressor (or a distance between the upper bent portion 140a and the lower bent portion 140b) and positions of a pair of perforations at which the vibration suppressor 140 is attached.